There exist different non-linear N-wave mixing optical systems, where N is a natural integer higher than or equal to three. In non-linear optics, the N-wave mixing covers in particular the three-wave mixing, where a transfer of energy occurs between three electromagnetic waves interacting in a non-linear medium, and the four-wave mixing, where a transfer of energy occurs between four electromagnetic waves interacting in a non-linear medium. The three-wave mixing finds, among other things, applications in the sum frequency, in particular the frequency doubling, the frequency differencing and the optical parametric amplification.
FIG. 1 schematically illustrates a conventional non-linear optical system configured for the optical frequency doubling, also called the second-harmonic generation. A first laser source 11 generates a first source signal 51 formed of light pulses at a given repetition frequency. A second laser source 12 generates a second source signal 52 formed of light pulses synchronized on the light pulses of the first source signal 51. The two laser sources 11, 12 have herein the same wavelength. The two light pulse signals 51, 52 are directed towards a non-linear optical medium 20. The non-linear optical medium may be a solid medium, for example an LBO, BBO, KDP crystal, or an optical fibre, or also a gaseous medium. The spatial and temporal superposition of the two source signals 51, 52 in the non-linear optical medium 20 makes is possible to generate an output signal 60 composed of light pulses at an optical frequency equal to the double of the optical frequency of the two laser sources 11, 12 and at the same repetition frequency as the two sources 11, 12.
FIG. 2 schematically illustrates another example of a non-linear optical system configured for the optical parametric amplification of light pulses. The same signs of reference represent components similar to those of FIG. 1. The two laser sources 11, 12 have generally the same repetition frequency, however the respective duration of the pulses is not necessary the same. Unlike FIG. 1, in FIG. 2, the two laser sources 11, 12 have generally not the same wavelength. A pump beam 52 composed of high-energy pulses is sent into a non-linear optical medium 20 to be combined with a signal beam 51 of lower energy. Under certain conditions, and in particular of spatial and temporal superposition of the pulses of the pump beam 12 and of the signal beam 11 in the non-linear optical medium 20, a transfer of energy may occur from the pump beam 12 towards the signal beam 11, hence generating an amplified beam 61 accompanied with the emission of a low-energy residual beam 62. This technique of optical parametric amplification hence makes it possible to amplify the signal 51 to form said amplified signal 61.
The different physical processes of N-wave mixing are instantaneous phenomena of non-linear optics that require the spatial and temporal superposition of all the pulses involved.
The shooting rate of the output pulse is often fixed as a function of the architecture used and of the features desired for the light pulse beam. The “Master Oscillator, Power Amplifier” (MOPA) architecture is commonly used to make high-power light pulse laser sources. In this case, the master oscillator of the laser source generates source pulses at a given rate, then these source pulses are amplified in one or more amplification stages.
Now, for certain applications, the user may need varying the pulse rate of the signal resulting from the non-linear optical conversion or even fully interrupting then resuming the shots.
The command or control of pulse train generated or amplified in a non-linear optical system may have different purposes. In some cases, it is desired to have a fast shutter, to switch-off or switch-on the beam from one pulse to the following one, at the output of the N-wave mixing system. In other cases, it is desired to have a frequency reducer, to generate an output signal at a repetition frequency lower than that of the oscillator of the laser source, for example by selecting one pulse out of 2, out of 3 or out of N. Moreover, it is desired to have a control system making it possible to generate a pulse on demand, the emission of a pulse being triggered on demand of the user via an electronic control signal.
A known technique consists in modifying the shooting frequency upstream from the non-linear optical conversion system. However, this frequency modification generally causes modifications of the features of the output beam (pulse energy and duration, shape and quality). In certain architectures, the modification of the shooting frequency before the non-linear optical (NLO) medium may produce super-power pulses and cause the degradation or even the destruction of the non-linear optical medium.
Another technique conventionally used to control the pulse train consists in placing an optical modulator, of the acousto-optic or electro-optic type, at the output of the non-linear optical conversion system. Such an optical modulator makes it possible, when activated, to modify the properties of the pulses that pass through it, so as to be able afterwards to spatially separate them from the other pulses. However, the speed of an electro-optic or acousto-optic modulator is currently limited to a rate of the order of 10 MHz. On the other hand, an optical modulator placed at the output of a non-linear optical conversion system induced power losses. These losses are of the order of 10 to 20% for an acousto-optic modulator and, respectively, of the order of 5% for an electro-optic modulator.
Finally, in the case of high-power lasers, the laser beam may damage the optical modulator if placed at the end of the amplification chain. Arranging an optical modulator at the output of a non-linear optical converter system hence suffers from significant limitations.